June 19, 2012
New Geology articles posted online ahead of print June 15
Orange-like rocks in Utah with iron-oxide rinds and fossilized bacteria inside that are believed to have eaten the interior rock material, plus noted similarities to "bacterial meal" ingredients and rock types on Mars; fine-tuning the prediction of volcanic hazards and warning systems for both high population zones and at Tristan da Cunha, home to the most remote population on Earth; news from SAFOD; and discovery in Germany of the world's oldest known mosses.Biosignatures link microorganisms to iron mineralization in a paleoaquifer
Karrie A. Weber et al., School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588, USA. Posted online 15 June 2012; doi: 10.1130/G33062.1.
Iron oxide rocks in an ancient aquifer give scientists clues about where to look for past life on Mars and other planets, including Earth. Scientists at the University of Nebraska-Lincoln and University of Western Australia have been studying rocks in Utah that resemble an orange with an iron cemented rind and an interior that consists of glued sand. These rocks formed millions of years ago in an ancient aquifer. Using microscopic methods, Karrie Weber and colleagues found tiny fossilized bacteria inside of these rocks, along with evidence corroborating that the bacteria were once alive inside the rock. Weber and colleagues think that these bacteria "ate" the iron in the rock to form the iron oxide mineral-rich rind. All of the ingredients for a bacterial meal exist on Mars and other areas on Earth. This has led the Weber and colleagues to theorize that similar iron-rich rocks could have been formed by bacteria and could still be forming today. The scientists are continuing to study how bacteria form rocks below Earth's surface so to better understand the conditions that support life and the signatures that life leaves behind. This research is supported by the University of Nebraska Research Office and Nebraska Tobacco Settlement Fund.
Relationship between dike and volcanic conduit distribution in a highly eroded monogenetic volcanic field: San Rafael, Utah, USA
Koji Kiyosugi et al., Dept. of Geology, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, USA. Posted online 15 June 2012; doi: 10.1130/G33074.1.
Cities like Auckland (NZ) and Mexico City are located within active volcanic fields, where new volcanoes will likely form in the future, creating a wide range of hazards. Where will these volcanoes form and what warning will residents have of impending eruptions? The geologic record preserved in old volcanic fields helps address this question. Koji Kiyosugi and colleagues studied magmatic system below an extinct volcanic field: the San Rafael subvolcanic field in Utah, USA. Below volcanoes, intrusive magma bodies formed before and during volcanic eruptions create vertical pipes (conduits) and vertical and horizontal sheets (dikes and sills, respectively). It is possible to observe these features of the magmatic system in great detail in this eroded volcanic field. Kiyosugi and colleagues mapped 63 conduits, ~2000 dike segments, and 12 sill complexes in the San Rafael. They find that the distribution of volcano conduits matches the major features of dike distribution, including development of clusters and distribution of outliers. These statistical models are then applied to the distributions of volcanoes in several recently active volcanic fields, where the distribution of intrusive magma bodies must be inferred from very sparse data. This comparison supports the use of statistical models in probabilistic hazard assessment for distributed volcanism.
Tristan da Cunha: Constraining eruptive behavior using the 40Ar/39Ar dating technique
Anna Hicks et al., Dept. of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ, UK. Posted online 15 June 2012; doi: 10.1130/G33059.1.
Tristan da Cunha (South Atlantic) is an active volcano and home to the most remote population in the world. The volcano last erupted in 1961, forcing the temporary evacuation of all 261 islanders. In an attempt to constrain eruptive behavior and better anticipate likely future activity, new 40Ar/39Ar ages were measured on 15 rock samples, carefully selected to reflect possible temporal correlations between eruptive style, composition, or vent location. All sample sites were precisely dated, including a very young deposit (3,000 plus or minus 1,000 years old). Results revealed no spatio-temporal pattern to activity at parasitic cones and recent volcanism from these eruptive centers varies in style, volume, and composition with time. Timing of a large-scale sector collapse was constrained to a 14,000-year window, and ages showed that the northern sector of the edifice was built very rapidly. It seems likely that the entire edifice was constructed piecemeal and has a far more complex evolution that previously assumed. Of particular significance to hazard assessment is the discovery that the summit was contemporaneously active with recent activity on the flanks and inhabited low lying coastal strips. The results present significant uncertainty in terms of anticipating future eruptive scenarios, and reflect the necessity for effective risk reduction measures on Tristan.
Bubble geobarometry: A record of pressure changes, degassing, and regassing at Mono Craters, California
James M. Watkins et al., Dept. of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA. Posted online 15 June 2012; doi: 10.1130/G33027.1.
Obsidian, natural volcanic glass, is one of the most recognizable rocks on Earth's surface. Obsidian exhibits a wide range in textures that record volcanic processes. For example, flow bands in obsidian and healed fractures provide field evidence that lava can break and then heal (like silly putty) in volcanic feeder systems. The orientation of bubbles and microscopic crystals can be used to infer obsidian flow dynamics and the timing and rates of crystallization. In this study, James M. Watkins and colleagues use new measurements on bubbles in obsidian to infer the pressure history of rising magma. Unlike bubbles that grow in an open can of soda, bubbles in magma can both grow and shrink as they rise toward Earth's surface. The study shows, for the first time, that the glass around bubbles preserves a record of physical changes in the magma feeder system prior to eruption. The measurements thus offer a new probe for inferring volcanic processes that are inaccessible to direct observation
Frictional properties and sliding stability of the San Andreas fault from deep drill core
B.M. Carpenter et al., Dept. of Geosciences and Energy Institute Center for Geomechanics, Geofluids, and Geohazards, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Posted online 15 June 2012; doi: 10.1130/G33007.1.
Experimental studies on samples collected from the actively slipping San Andreas Fault, as part of San Andreas Fault Observatory at Depth (SAFOD) drilling in central California, USA, have provided important new insights into the mechanics and slip behavior of the fault at depth. B.M. Carpenter and colleagues report, for the first time, on the frictional properties of intact fault rock samples recovered from seismogenic depths. Their results explain several fundamental and longstanding observations along the San Andreas fault, including (1) the inferred extreme mechanical weakness and creeping behavior of the active fault in central California; (2) the occurrence and observed stress drop of repeating micro-earthquakes on faults to the northeast of the actively creeping fault strand; and (3) highly localized fault weakness, as documented by an extraordinarily sharp transition from frictionally weak fault rock within the main creeping strand of the San Andreas fault to stronger wall rock more than a mile away.
Subsidence of the West Siberian Basin: Effects of a mantle plume impact
Peter J. Holt et al., Geospatial Research Ltd., Durham University, Durham DH1 3LE, UK. Posted online 15 June 2012; doi: 10.1130/G32885.1.
Comparison of computer modeling results with the observed subsidence patterns from the West Siberian Basin provides new insight into the origin of the Siberian Traps flood basalts and constrains the temperature, size, and depth of an impacting mantle plume head during and after the eruption of the Siberian Traps at the Permian-Triassic boundary (250 million years ago). Peter J. Holt and colleagues compare subsidence patterns from a one-dimensional model of conductive heat flow to observed subsidence calculated from studies of the sediments in the basin. This results in a best-fit scenario with a 50-km-thick initial plume head with a temperature of 1500 degrees Celsius situated 50 km below the surface, and an initial regional crustal thickness of 34 km, which is in agreement with published values. The observed subsidence and modeling results agree very well, including a 60-90-million-year delay between the eruption of the flood basalts and the first regional sedimentation. These results reemphasize the viability of a mantle plume origin for the Siberian Traps, provide important constraints on the dynamics of mantle plume heads, and suggest a thermal control for the subsidence of the West Siberian Basin.
The relationship between surface kinematics and deformation of the whole lithosphere
L. Flesch and R. Bendick, Dept. of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907-2051, USA. Posted online 15 June 2012; doi: 10.1130/G33269.1.
There has been a proliferation of geoscience research efforts over the past decade because of the new understanding of how the surface of the continents are moving using GPS surface observations. This data has been used either to asses forward numerical models of lithospheric or constrain inversions for the dynamics of continents to identify the forces driving the observed deformation. Such efforts inherently assume that information about dynamics is efficiently transferred from lithospheric depths to the surface. This is indisputably the case for oceanic lithosphere, but L. Flesch and R. Bendick show that it applies only in a limited subset of the plausible mechanical strength configurations for continents. Making such an assumption in other cases results in unreasonable conclusions about either the mechanical properties of the lithosphere or incorrectly complicated heterogeneous balance of forces.
Variability in the length of the sea ice season in the Middle Eocene Arctic
Catherine E. Stickley et al., Dept. of Geology, University of TromsÃ¸, N-9037 TromsÃ¸, Norway. Posted online 15 June 2012; doi: 10.1130/G32976.1.
Finely laminated marine sediments of middle Eocene age (about 45 million years old) are preserved along the Lomonosov Ridge in the central Arctic. These sediments comprise two main components: (1) those indicative of sea ice (fossil species of the delicate, sea ice-dwelling diatom Synedropsis spp.[siliceous microfossils]) and sea ice-rafted debris (sea ice-IRD); and (2) those indicative of open marine conditions (e.g., other diatom taxa and siliceous microfossil types). Their coexistence strongly implies seasonality, but to know with certainty, the annual flux cycle must be reconstructed. For the first time, Catherine E. Stickley and colleagues use a non-destructive technique to resolve and reconstruct seasonal-scale flux events from these sediments. They reveal discrete productivity-flux events at ultra-high (e.g., about 30 microns) resolution and show that seasonality is expressed at the submillimeter scale by successions of discrete mono-specific laminae and micro-lenses of Synedropsis species, of sea ice-IRD, and of open-water taxa. These findings indicate that first-year winter sea ice existed in the Arctic during the middle Eocene. A preliminary assessment of annual cycles shows that suborbital variability existed on the order of multi-decadal to centennial duration. Stickley and colleagues argue that this reflects variations in the sea ice season length. Past records at such time scales are especially important because they may reveal patterns of Earth system behavior of direct relevance to modern observations of Arctic change.
Oldest known mosses discovered in Mississippian (late Visean) strata of Germany
Maren HÃ¼bers and Hans Kerp, Forschungsstelle fÃ¼r PalÃ¤obotanik, Institut fÃ¼r Geologie und PalÃ¤ontologie, WestfÃ¤lische Wilhelms-UniversitÃ¤t MÃ¼nster, Schlossplatz 9,48143 MÃ¼nster, Germany. Posted online 15 June 2012; doi: 10.1130/G33122.1.
Today bryophytes, with about 20,000 species of hornworts, liverworts, and mosses, are the most diverse group of non-vascular land plants. Mosses are important constituents of terrestrial ecosystems, from the tropics to the high latitudes. In many modern wetland ecosystems, mosses play a major role in nutrient cycling and water storage. Molecular clock data indicate that mosses appeared before the first vascular land plants, but their fossil record is extremely poor. Carboniferous wetland environments, with their unequaled accumulation of plant biomass, likely provided ideal habitats for mosses. Coal floras have been studied in great detail, but the fossil record of Carboniferous mosses is remarkably meager, though it should be noted that mosses are often difficult to recognize. Three types of mosses showing cellular preservation have now been identified from approx. 330 million-year-old rocks from eastern Germany. These are the oldest unequivocal mosses known to date, and even though the remains are small, they demonstrate that mosses formed part of Carboniferous ecosystems. The moss fossils were obtained from organic residues after whole-rock samples had been dissolved. This method, which is now rarely used for studying Carboniferous flora, may reveal that mosses were more widespread than commonly thought.
A detailed record of shallow hydrothermal fluid flow in the Sierra Nevada magmatic arc from low-delta-18O skarn garnets
Megan E. D'Errico et al., Dept. of Geology, Trinity University, San Antonio, Texas 78212, USA. Posted online 15 June 2012; doi: 10.1130/G33008.1.
Garnet from skarns exposed at Empire Mountain, Sierra Nevada (California, USA) batholith have variable delta-18O values, including the lowest known delta-18O values of skarn garnet in North America. Such values indicate that surface-derived meteoric water was a significant component of the fluid budget of the skarn-forming hydrothermal system, which developed in response to shallow emplacement (~3.3 km) of the 109 million year old quartz diorite of Empire Mountain. Brecciation in the skarns and alteration of the Empire Mountain pluton suggests that fracture-enhanced permeability was a critical control on the depth to which surface waters penetrated to form skarns and later alter the pluton. Compared to other Sierran systems, much greater volumes of skarn rock suggest an exceptionally vigorous hydrothermal system that saw unusually high levels of decarbonation reaction progress, likely a consequence of the magma intruding relatively cold wall rocks inboard of the main locus of magmatism in the Sierran arc at that time.
The influence of a mantle plume head on the dynamics of a retreating subduction zone
Peter G. Betts et al., School of Geosciences, Monash University, Clayton, VIC 3800, Australia. Posted online 15 June 2012; doi: 10.1130/G32909.1.
Earth subduction zones are where two geological plates of the outer Earth converge and the dense ocean crust sinks into the mantle. Subduction zones form an important component of mantle convection. Mantle plumes are hot buoyant material that rises from deep in the interior of the Earth and interact with the Earth's crust. When plumes interact with ocean crust they can form large areas of buoyant ocean floor topography. Subduction zones can migrate backward and interact with mantle plumes, causing the subduction zone to change behavior. Peter G. Betts and colleagues modeled this geological situation and have discovered that subduction zone/plume interactions can cause massive geological damage at the edges of plates. The buoyant plume head hinders subduction and causes the subduction zone to migrate forward causing intense deformation in the adjacent geological plate. The subducting oceanic plate is also damaged and large tears can form allowing the plume to migrate across plate boundaries. The Yellowstone hotspot may be an ancient example of this process.
On the Net: